Joint assemblies on structures, particularly aircraft and other vehicle structures and assemblies often join components made from dissimilar materials. For example, joint assemblies typically operate to join components made from similar or dissimilar materials. When dissimilar materials are to be joined, composite materials may, for example, be joined to metal-containing components. When joining components made from dissimilar materials, such as a component made from a composite material joined to a component made from a metal such as, for example, aluminum, a large difference in electrical potential is created. Such differential in electrical potential can increase the probability of corrosion of, for example, aluminum, especially when oriented proximate to composite components having exposed carbon fibers edges. Any flaws in the edges or ends of the composite material or the sealant covering the edges or ends can exacerbate the collection of, or otherwise serve as sites to retain moisture that may build up within joints used to connect the components (e.g. butt joints). Such moisture that is then subjected to varying temperatures can potentially degrade the ends or edges of an exposed composite component. Moreover, if moisture is allowed to remain in contact with ends or edges of joined metal components, such as aluminum components, corrosion can occur.
In the aerospace field, including the manufacture of aircraft, it may be desired to join composite components with metal components via joining assemblies. The joining of dissimilar components can offer particular advantages, as the properties of both materials may be desirable for a particular assembly.
The joining of dissimilar components in a joint assembly can present challenges as the dissimilar materials may react differently to various environment conditions. For example, the dissimilar materials may react differently to moisture build-up that could lead to the corrosion of the metal components. While the CFRPs themselves demonstrate good corrosion resistance, moisture may build up and present undesirable effects at and within a joint assembly. Structures comprising joined assemblies can experience wide ranging temperatures that can lead to degradation of certain mechanical properties of the CFRPs, such as, for example, at the matrix-fiber interface and at the edges of the composite components where the fibers may be exposed to and retain amounts of moisture. When composite components are joined in close proximity to metal components, such as in a common joint (e.g. joining of dissimilar materials), even though the CFRPs themselves may not be affected by the moisture present, the carbon fibers in the CFRPs can contribute to galvanic corrosion in the commonly-joined metal components.
Galvanic corrosion is caused by dissimilar metals and alloys have different electrode potentials coming into contact through an electrolyte (e.g. moisture/water, especially if salt or industrial pollutants are present) with one metal acting as anode and the other metal acting as a cathode. The electrode potential difference between the dissimilar metals results in an accelerated attack on the anode member of the galvanic couple. The anode metal dissolves into the electrolyte, and a deposit collects on the cathodic metal. In the case of moisture build-up, water can become an electrolyte, thereby providing a means for ion migration whereby metallic ions move from the anode to the cathode within the electrolyte. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited.
Such corrosion, including galvanic corrosion of joined metal components, can necessitate the replacement of such metal components, resulting in increased cost as the structure comprising the corroding metal component must be removed from service while the metal component is serviced or replaced.